Complete and uniform saturation of myocardium is essential for quantitative myocardial perfusion imaging using the first pass of a contrast agent. At 3 T, inhomogeneities of both the static (B0) and radiofrequency (B1) magnetic fields have led to the use of adiabatic B1-insensitive rotation type 4 (BIR-4) pulses, which in practice are constrained by radiofrequency (RF) heating.
First-pass magnetic resonance (MR) myocardial perfusion imaging (MPI) is an established technique for the assessment of ischemic heart disease. Saturation recovery preparation is widely used to produce T1-weighted images rapidly and with multiple slice coverage. Contrast-to-noise ratio (CNR) of myocardial wall enhancement during the first pass is critical to the performance of this technique. Recent work at 3 T has demonstrated improved CNR and showed that the diagnostic performance of 3-T MPI is superior to that of 1.5 T for the identification of both single- and multiple-vessel coronary disease. Despite the gain in CNR, there are still major technical challenges in 3-T cardiac imaging, such as static (B0) and radiofrequency (B1) magnetic field inhomogeneities.
Qualitative and quantitative MPI both rely on complete and uniform saturation of myocardium, and the performance of saturation pulses is sensitive to variations in the B0 and B1 fields. Recent studies have compared the effectiveness of different saturation pulses and have shown that the rectangular RF pulse train and the adiabatic B1-insensitive rotation type 4 (BIR-4) pulse exhibit better saturation effectiveness than the conventional 90° rectangular hard pulse with 1.5-T and 3-T cardiac imaging. The conventional RF pulse train, however, is more susceptible to B1 inhomogeneity than the BIR-4 pulse. Conversely, the BIR-4 pulse has a higher specific absorption rate (SAR). These costs inherently limit their application at 3 T, where low RF power deposition and immunity to B0- and B1-field inhomogeneities are highly desirable. At 3 T, inhomogeneities of both the static (B0) and radiofrequency (B1) magnetic fields have led to the use of adiabatic B1-insensitive rotation type 4 (BIR-4) pulses, which in practice are constrained by radiofrequency (RF) heating.